How to Avoid the 5 Worst Interior Design Mistakes

AFTER 16 MONTHS cooped up with roommates or parents, young (and not-so-young) people have had enough. Those who can afford it are increasingly moving into their own first places when their leases end this summer and autumn, said a spokesman for real-estate rental site StreetEasy. Searches that specified studio apartments are up 69% year-over-year.

When it comes to decorating these solo nests, however, designers say first timers’ greenness leads to errors: from cramming oafish sofas through doors they failed to measure to living sans civilities like curtains and rugs. As New York City designer Phillip Thomas said, “Just because it’s your first apartment doesn’t mean it can’t have a sense of sophistication.”

Here, design pros highlight the five flubs that novice renters most frequently make on their way to, as millennials call it, “adulting.” Plus: chic alternatives.

1. The Unconquered Divide

Generations of squished people have passed down various methods to separate a studio apartment into living and sleeping spaces: curtains, free-standing screens, bookshelves, even a delineating row of jungle-y plants. They all can make a space feel smaller, said Francesca Bucci, founder of BG Studio in Manhattan. Mr. Thomas noted that such barriers frequently cut off window light, creating a murky cave. “There is nothing more awful than living in a space without light,” he said.

Instead: Rather than placing your bed’s headboard against a wall, Ms. Bucci directed, “float” the bed, with the foot facing a window and leaving at least two feet of circulation at the bottom. A medium-height headboard will act as a divider without depriving the rest of the studio of natural light. Arrange your seating area on the other side of it, backing your couch against it. This way you won’t subject guests to your rumpled pillows or that stuffed animal from which you haven’t managed to brutally sever ties just yet.

2. Helter-Skelter Inheritances

Beware a hodgepodge of hand-me-down furniture relatives have cast off. Manasquan, N.J., designer Christina Kim warned that, “the scale of such furniture is usually off, and a mix of too many styles can feel chaotic.”

Instead: “Do not feel obligated to accept every piece that comes your way,” said Mr. Thomas. If a donation doesn’t work with your décor, politely decline it or modify the offering so it suits your style. In his first rental, in Washington, D.C., Mr. Thomas draped quilts and tossed cut-velvet pillows to align random sofas with his aesthetic.

3. Place-Holder Art

Worried about forfeiting security deposits, renters often settle for a few posters hung with adhesive strips, complained New York City designer Young Huh. Even with more ambitious prints or paintings, noted fellow Manhattan designer Starrett Ringbom, newbies tend to hang them too high, mounted in cheap plastic frames.

Instead: Invest in some spackle. “Patching and painting at the end of the lease—even if only a year—is a small price to pay for an inviting and collected home,” Ms. Huh said. Hang art at eye level for comfortable viewing, advised Ms. Ringdom, who also contends that having art professionally framed is a worthwhile investment. “A silver-leaf frame instantly elevates a poster from your last museum visit into art worthy of the living room wall,” she said.

4. Single-Source Sameness

“It’s so exciting getting your first place, and often you’ll shop for everything at once from the same big-box store,” said Lauren Wall, co-founder of Principle Faucets, in Santa Cruz, Calif. But can a single retailer really represent your many-faceted personality?

Instead: “Invest some time in searching for killer, high-quality resale pieces to mix with budget-friendly new items,” Ms. Wall suggested. Your space will have “more intention and character” than if you buy everything at once. Mr. Thomas recommended searching estate sales and online auctions. And don’t just fixate on how a particular piece looks in the context of a catalog photo: Catalina Echavarria, co-founder of Miami furniture and interior design firm CEU Studio, suggested you shop in person, if possible, and think about how you’ll use the item. “If I sit on a couch, I want to feel hugged and nurtured…if I step on a rug, I want to love it barefoot and feel its texture,” she said.

5. Casting a Bad Light

If you think you’re all set with your landlord’s flush-mounted ceiling lights (aka “boob lights,” so christened because they often take the form of hemispheres of milky glass with nipple-like finials), think again. “Overhead lighting is unflattering and ineffective for tasks such as reading,” said Washington, D.C., designer Annie Elliott, who pointed out that these fixtures often use bulbs that cast white walls in eerie, blue-ish, hospital-like glows.

Instead: Buy a cheerful table lamp to add color, style and, of course, light, said Ms. Elliott. “It will elevate the entire room.” Warm, yellow-toned lightbulbs will help create a homey feeling. Swap out your landlord’s ceiling bulbs and store them so you can replace them when you move out.

ODD SQUAD

Pros recall weird first-time decorating moves

“Beach chairs as lounge chairs. Keep the outdoor furniture outside!” — Marc Bacher, founder, Stuga, Austin, Texas

“Beer cans stacked to create a base and a piece of glass on top. Creative way to recycle but not a good look when you are trying to look grown-up. I’ve also seen bed sheets nailed to the window frame as curtains.” — Amanda Thompson designer, New York City

“A shelf of glass bottles filled with highlighter-infused water to display with black lights. Actually, just say no to black lights to begin with.” — Lauren Wall, co-founder, Principle Faucets, Santa Cruz, Calif.

“A contractor’s work light, with the plastic cage, draped over a bookcase.” — Annie Elliot, interior designer, Washington, D.C.

“Furniture fashioned out of cinder blocks. It was ominous.” — Christina Kim, interior designer, Manasquan, N.J.